In general, a marine structure is put in motion by external forces, such as waves, tides and winds, and such motion has to be reduced for reasons of performance and strength of equipment and attachments of the marine structure. To achieve this, the submerged portion of floating structures is elaborately designed in various ways. For example, according to a method, the waterplane area of the float is reduced, and the displacement of the submerged portion beneath the waterplane is increased, thereby reducing motion of the structure caused by waves having periods within a certain range. Floating structures based on this method is referred to as semi-submersible type, and the method is used for oilrigs, marine crane barges, pipe laying barges, production platforms, large offshore structures (such as offshore airports) and the like.
FIG. 6 are diagrams showing a semi-submersible oilrig. FIG. 6(A) is a schematic front view of the semi-submersible oilrig, and FIG. 6(B) is a side view of a floating structure of the semi-submersible oilrig. The floating structure of the semi-submersible oilrig has lower hulls 1 that provide a displacement under the water surface, an upper hull 2 that supports upper facilities, such as a machine room, an accommodation space and a rig, above the water surface, columns 3 that have a small cross section and couple the lower hulls 1 and the upper hull 2 to each other, and braces 4 that three-dimensionally couple these components to each other. In addition, FIG. 6(A) shows a derrick 6 that supports a drill pipe with a cutter for digging in the sea bottom 5 and a riser pipe 7 for circulating muddy water, which are main outer components of the semi-submersible oilrig.
A typical semi-submersible oilrig has two lower hulls 1 that constitute a float as shown in FIG. 6(A), has two to four columns 3 (four columns 3 in the drawing) on each lower hull as shown in FIG. 6(B), and has a plurality of braces 4 forming a truss structure below the upper hull 2 and between the opposing columns 3 as shown in FIG. 6(A). The lower hull 1 may have tapered ends to reduce the resistance when the semi-submersible oilrig is moved or towed. In general, semi-submersible oilrigs in their infancy have three to five columns interconnected by braces and mounted on an upper hull and separate floats, referred to as footings, connected to a lower portion of each column.
Such a semi-submersible oilrig moves floating with the lower hulls 1, and the lower hulls 1 and some of the columns 3 are filled with ballast water to make the oilrig sub-merged at the destination, thereby allowing the semi-submersible oilrig to conduct the digging operation at one fixed place in the ocean. In general, the draft is designed to prevent the bottom of the upper hull 2 from being washed by waves. Therefore, the braces 4 intersect the draft line, and external forces, such as a splitting force (a force to separate the lower hulls in the lateral direction), a pitch connection moment (a moment to make the lower hulls pitch out of phase with each other by 180 degrees) and a racking force (a force to move the lower hulls out of phase with each other by 180 degrees in the longitudinal direction), are exerted on the braces. Thus, the braces 4 and the joints are susceptible to damage from repeated applications of loads, such as waves. Such damage leads to collapse of the rig, and therefore, the lifetime of the semi-submersible oilrig depends on the durability of the braces 4.
As described above, the braces 4 are important to ensure the strength of the semi-submersible floating structure and therefore have to be maintained at regular intervals. Typically, the maintenance is performed on the ocean or in a dock by exposing the braces 4 above the water surface by discharging the ballast water. When the maintenance is performed in a dock, burdens or upper facilities on the upper hull 2 may be removed to reduce the total weight before the braces 4 are exposed above the water surface. There is a problem that the floating structure cannot be used during the maintenance. Thus, there is a demand that the durability of the floating structure is increased to minimize the frequency of maintenances of the braces 4 of the floating structure in operation.
In addition, although the semi-submersible floating structure has conventionally been used in areas of depths D between 300 and 500 m, recently, there is a growing demand that the semi-submersible floating structure is used in very deep water of 1500 m to 2000 m. To use the semi-submersible oilrig in very deep water, a longer drill pipe and a longer riser pipe 7 are needed, and therefore, the load on the upper hull 2 (the variable deck load) increases. For example, when the depth D is 300 to 500 m, the variable deck load is about 2000 to 2500 t. However, when the depth D is 1500 to 2000 m, the variable deck load is 4000 to 5000 t or more. As a result, the conventional floating structure cannot have sufficient buoyancy and therefore cannot be used without modification.
A method for increasing the buoyancy of a floating structure is to install an auxiliary float on a column or a lower hull. For example, in Japanese Patent Laid-Open No. 2001-180584 (patent literature 1), there is described an invention in which each column of a floating structure has an additional floating portion having a larger cross section at the level of the draft line. Furthermore, a method in which a box-shaped auxiliary float is installed on each lower hull and welded to a column and the lower hull and a method in which an auxiliary float is installed on the perimeter of each lower hull are also known.    Patent literature 1: Japanese Patent Laid-Open No. 2001-180584